This invention relates to pipe lining apparatus wherein a protective coating of mortar or similar material in plastic condition is applied to the interior of a pipe or other cylindrical conduit and more particularly to a troweling means for smoothing the surface of such an interior coating directly after the same has been applied to or deposited against the interior wall of a pipe. Such troweling means are generally known in the art as drag trowels and are usually frustoconical, having small leading ends and larger trailing ends. The trowels are commonly connected to the rear ends of mortar applying machines to be drawn through the pipe thereby.
The prior art contains a number of examples of sheet metal frustoconical trowels for the general purpose of smoothing an interior mortar coating by pulling the trowel through a pipe wherein such mortar has been applied to the interior surface of the pipe. The trowel is drawn through the pipe with its small end first and the large or trailing end of the trowel which performs the mortar-smoothing function is necessarily resilient and must be readily expansible and contractable in a diametral direction while still generally retaining its circular form or tending to maintain such form even though local conditions may cause the large troweling end of the trowel to assume somewhat elliptical or other distorted forms temporarily.
Since trowels of this type are drawn through a pipe under conditions where the operation of the trowel cannot be observed during its functional periods, the necessity for safe, accurate and foolproof operation and self-adjustment for various physical conditions which are encountered by the trowel in its passage through a pipe is of great importance.
The present invention is concerned particularly with the means for applying resilient expansive forces to the large trailing end portions of frustoconical trowels of this general type. A considerable variety of trowel expanding arrangements are found in the prior art. Representative examples will be found in Perkins U.S. Pat. Nos. 2,924,867 and 3,188,710 and Ruegsegger U.S. Pat. Nos. 3,257,698 and 3,257,699. Another example of a trowel expansion means of the present invention is found in Barton U.S. Pat. No. 3,263,296 wherein an expansion coil spring is disposed within the large end of the trowel at and extending across each overlap of the trowel plate or plates for applying expansive forces thereto. Such a coil spring is, by its nature, not readily adjustable as to the expansive force which it exerts and, furthermore, is apt to become fouled by dried mortar so that its operation is not uniform and reliable and may in certain cases be more or less entirely useless by reason of the presence of dried mortar in the coils of the spring or other operating parts.
It is believed that the closest approaches in the prior art are found in Perkins U.S. Pat. No. 3,188,710 and Barton U.S. Pat. No. 3,368,253 and the Ruegsegger patents enumerated above.